Method of creating defect free high Ge content (&gt; 25%) SiGe-on-insulator (SGOI) substrates using wafer bonding techniques

ABSTRACT

A method for achieving a substantially defect free SGOI substrate which includes a SiGe layer that has a high Ge content of greater than about 25 atomic % using a low temperature wafer bonding technique is described. The wafer bonding process described in the present application includes an initial prebonding annealing step that is capable of forming a bonding interface comprising elements of Si, Ge and O, i.e., interfacial SiGeO layer, between a SiGe layer and a low temperature oxide layer. The present invention also provides the SGOI substrate and structure that contains the same.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/939,736, now U.S. Pat. No. 7,235,812 filed Sep. 13, 2004.

The present invention described herein was made with U.S. Governmentsupport under Contract No. N66001-00-C-8086 awarded by The Departnent ofthe Navy. The U.S. Government thus has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to the fabrication of a SiGe-on-insulator(SGOI) substrate, and more particularly, to a substantially defect free,high Ge content (>25 atomic %) SGOI substrate as well as method offabricating the same. The method of the present invention includes awafer bonding technique that is capable of forming an interfacialbonding layer that includes elements of Si, Ge and O. The interfacialbonding layer is referred to herein as an interfacial SiGeO layer.

BACKGROUND OF THE INVENTION

It is well known to those skilled in the art that strained silicon canenhance carrier mobility for both electrons and holes in comparison withbulk silicon. In addition, the degree of mobility enhancement stronglydepends on the strain level in the strained silicon layer. Namely, thehigher the imposed “tensile” strain, the higher the enhancement onmobility. The most common practice for applying or inducing tensilestrain to a silicon layer is through the use of an underlying silicongermanium (SiGe) buffer layer, which is typically a relaxed SiGe layerhaving a larger lattice constant as compared to bulk silicon. Hence, byincreasing the Ge content of the underlying SiGe buffer layer, which inturn increases the lattice constant of the SiGe buffer layer, a higher“tensile” strain can then be imposed to the silicon layer due to alarger lattice mismatch between the two layers.

It is also well known that metal oxide semiconductor field effecttransistor (MOSFET) devices fabricated on silicon-on-insulator (SOT)substrates can have up to 25-35% better performance than those built onbulk Si wafers due to lower parasitic capacitance of the source/drainjunction, reduced short channel effects and better device isolation.This is reported, for example, in G. G. Shahidi, “SOI Technology for GHzEra”, IBM J. Res. & Dev., Vol. 46, pp. 121-131 (2002). Thus, it isdesirable to combine these two effects to generate a strained siliconlayer having enhanced carrier mobility on a SiGe-on-insulator (SGOI)substrate to achieve an even higher device performance gain.

However, this prior scheme is faced with two major problems or issues inits attempt to obtain the best device performance from this synergisticcombination of strained silicon on SGOI. One issue is the ability togenerate a fully relaxed SiGe buffer layer with a high Ge content suchthat its lattice constant is strictly determined by the value of the Gecontent and is independent of its degree of relaxation. Otherwise, theimposed “tensile” strain to the silicon layer would not be as high asdesired. Moreover, in the case for a partially relaxed SiGe buffer, theimposed strain could easily fluctuate or change due to subsequentthermal processing or integration steps which is undesirable.

The second issue is the ability to create a near defect-free SGOIsubstrate with a high Ge content, which is the more difficult problem toaddress. Although thermally-mixed (TM) SGOI has provided an alternativeapproach to creating a SGOI substrate, the SiGe layers typically formedon the TM-SGOI wafers are partially relaxed, i.e., about 50 to 65% atbest, and have yet to achieve a fully relaxed SiGe layer with greaterthan 90% relaxation.

In a similar way, the same difficulty has been encountered inpreparation of SGOI substrates generated by the alternate SIMOXapproach. See, for example, T. Mizuno, et al.,“High PerformanceStrained-Si p-MOSFETs on SiGe-on-Insulator Substrates Fabricated bySIMOX Technology”, IEDM Tech. Dig., pp. 934-936 (1999).

Recently, it has been demonstrated that a fully relaxed SiGe bufferlayer can be transferred to a handle wafer through a wafer bondingtechnique. See U.S. Pat. No. 6,524,935 to D. F. Canaperi, et al.However, the bonded SGOI wafers prepared from this prior art processstill suffer from various bond-induced defects, such as blisters,bubbles, voids, etc., especially for the high Ge content SGOI waferswhere the Ge content is larger than 25 atomic (at.) %.

SUMMARY OF THE INVENTION

The present invention describes a method for achieving a substantiallydefect free SGOI substrate which includes a SiGe layer that has a highGe content of greater than about 25 atomic % using a low temperaturewafer bonding technique. The term “substantially defect free” is used inthe present application to denote a SiGe layer that has a defectdensity, including misfits, threading defects, microtwins, stackingfaults and other defects, that is about 10⁴ to about 10⁵ defects/cm² orless, and is dictated by the initial SiGe buffer. Currently, hightemperature RT-CVD growth techniques are being developed to grow highcontent SiGe buffers with low defect densities in the range of 10³-10⁴defects/cm². The wafer bonding process described in the presentapplication includes an initial prebonding annealing step that iscapable of forming a bonding interface comprising elements of Si, Ge andO, i.e., an interfacial SiGeO layer, between the SiGe layer and anoxide, which substantially eliminates or reduces any bonding induceddefects and increases the bonding yield of the SGOI substrates.

Specifically, by subjecting an initial high Ge content SiGe wafercovered with a layer of deposited low temperature oxide (later servingas the buried oxide, BOX) to an appropriate thermal annealing step, abonding interface comprising a thin layer of SiGeO or a mixture of Si/Geoxide is formed in the present invention between the SiGe layer and thelow temperature oxide. This interdiffused or oxygen-enriched SiGeOlayer, i.e., the bonding interface or interfacial SiGeO layer, isbelieved to act as a gettering layer to either trap defects or preventvolatile gases or residuals (such as hydroxyl group, hydrogen, oxygen,and the like) from penetrating into the SiGe film from either the BOXlayer or the bonded BOX/SiGe interface during a densificaton step andcan thereby reduce or eliminate any undesirable blistering due toentrapped, residual volatile species.

In addition, the bonding interface layer, i.e., the interfacial SiGeOlayer, can further serve to suppress hydrogen from diffusing ormigrating from the high concentration region (of the smart cut region)to the SiGe/BOX interface during the subsequent post-bonding anneal andwafer splitting anneal steps. If no such anneal step is implemented andno such interfacial layer exists, the bonded SGOI with its high Gecontent layer will suffer from severe blistering, bubbling, and voidgeneration problems typically associated with a low yield in waferbonding process. Nevertheless, the thickness of the interfacial SiGeOlayer can be tailored by adjusting the anneal temperature and time.

In broad terms, the method of the present invention comprises the stepsof:

forming a low temperature oxide atop a structure that comprises a fullyrelaxed SiGe layer located on a sacrificial substrate;

first annealing said structure including said low temperature oxide at afirst temperature to form an interfacial layer comprising elements ofSi, Ge and O between the low temperature oxide and said SiGe layer;

providing an implant region within said fully relaxed SiGe layer;

bonding said low temperature oxide to a surface of a semiconductorsubstrate, wherein said bonding comprises contact bonding to form a bondbetween said exposed surface of said low temperature oxide and saidsemiconductor substrate, a second anneal at a second temperature tostrengthen said bond, and a third anneal performed at a thirdtemperature that is greater than the second temperature to causeseparation at said implant region within said fully relaxed SiGe layer,whereby said sacrificial substrate and a portion of the fully relaxedSiGe layer are removed; and

re-annealing the structure at a fourth temperature that is greater thanthe third temperature to form a SiGe-on-insulator (SGOI) substrate thatcomprises the semiconductor substrate, said low temperature oxidelocated on said semiconductor substrate, and said fully relaxed SiGelayer having a defect density of about 10⁴ to about 10⁵ defects/cm² orless and a Ge content that is greater than 25 atomic % located atop saidlow temperature oxide, wherein said low temperature oxide and said fullyrelaxed SiGe layer are separated by said interfacial layer.

In some embodiments, the fully relaxed SiGe layer can be smoothed andthinned after the re-annealing step. In yet another embodiment of thepresent invention, a thin SiGe buffer layer can be grown atop the fullyrelaxed SiGe layer after the re-annealing step and a strainedsemiconductor layer can be formed thereon. Alternatively, a strained Sior SiGe semiconductor layer can be formed directly on the fully relaxedSiGe layer after the re-annealing step. At least one complementary metaloxide semiconductor (CMOS) device such as a nFET or a pFET can be formedatop the strained semiconductor layer utilizing conventional CMOSprocessing steps that are well known in the art.

In another embodiment, a high temperature oxide is formed on the fullyrelaxed SiGe layer prior to formation of the low temperature oxide.

In addition to the processing steps mentioned above, the presentinvention also provides a SiGe-on-insulator (SGOI) substrate thatcomprises

a semiconductor substrate;

a buried oxide layer located on said semiconductor substrate; and

a fully relaxed SiGe layer having a defect density of about 10⁴ to about10⁵ defects/cm² or less and a Ge content that is greater than 25 atomic% located atop said buried oxide layer, wherein said buried oxide layerand said SiGe layer are separated by an interfacial layer comprisingelements of Si, Ge and O.

The present invention also provides a semiconductor structure thatincludes:

a strained semiconductor layer located atop a SiGe-on-insulator (SGOI)substrate, said SGOI substrate comprising a semiconductor substrate, aburied oxide layer located on said semiconductor substrate, and a fullyrelaxed SiGe layer having a defect density of about 10⁴ to about 10⁵defects/cm² or less and a Ge content that is greater than 25 atomic %located atop said buried oxide layer, wherein said buried oxide layerand said fully relaxed SiGe layer are separated by an interfacial layercomprising elements of Si, Ge and O.

It is noted that the term “fully relaxed” when used in conjunction withthe SiGe layer denotes a layer of silicon germanium that has a measuredrelaxation value of greater than 90%. More preferably, the fully relaxedSiGe layer of the present invention has a measured relaxation value ofgreater than 95%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E are pictorial representations (through a cross sectionalviews) illustrating the processing steps that are employed in thepresent invention for fabricating a substantially defect free, high Gecontent SGOI substrate.

FIG. 2 is a cross-sectional view of the SGOI substrate shown in FIG. 1Eafter forming a strained semiconductor layer on the fully relaxed SiGelayer.

FIG. 3 is an actual TEM cross sectional micrograph showing an SGOIsubstrate created using the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which provides a substantially defect free, highGe content SGOI substrate and a method of fabricating the same, will nowbe described in greater detail by referring to the following descriptionand drawings that accompany the present application. It is noted thatthe drawings depicted in FIGS. 1A-1E and FIG. 2 are provided forillustrative purposes and thus those drawings are not drawn to scale.

Reference is first made to FIG. 1A which shows an initial structure 10that is employed in the present invention. The initial structure 10includes a SiGe layer 14 that is crystalline and filly relaxed locatedon a surface of a sacrificial substrate 12. The sacrificial substrate 12can include any type of material including semiconducting, insulating orconducting, preferably semiconducting or insulating, and even morepreferably semiconducting. Illustrative examples of semiconductingmaterials that can be used as the sacrificial substrate 12 include, butare not limited to: Si, SiGe, SiC, SiGeC, GaAs, InP, InAs and layeredsemiconductors including, for example, Si/SiGe, SOIs and SGOIs. Examplesof insulating materials that can be employed as the sacrificialsubstrate 12 include various glasses or polymers, and examples ofconducting materials that can be employed as the sacrificial substrate12 include metals.

In accordance with the present invention, the SiGe layer 14 is a gradedlayer that includes a varying content of Ge in which the initial contentof Ge that is in proximity to the sacrificial substrate 12 is low (10atomic % or less, preferably 5 atomic % or less). The Ge content is thenincreased within the SiGe layer 14 in a stepwise manner, graduallyreaching the intended high Ge content. By “high Ge content” it is meanta SiGe layer having a Ge content that is greater than 25 atomic %,preferably greater than 30 atomic %, more preferably greater than 35atomic %, and even more preferably greater than 45 atomic %. The gradedportion of the SiGe layer 14 is formed utilizing a conventionaldeposition process such as, for example, ultra-high vacuum chemicalvapor deposition (UHVCVD) or rapid thermal chemical vapor deposition(RTCVD), that is well known to those skilled in the art. The gradedportion of the SiGe layer 14 has a thickness that is typically fromabout 100 to about 1500 nm.

After the target Ge content is reached, a thick upper relaxed SiGebuffer portion is formed over the graded portion providing the SiGelayer 14 shown in FIG. 1A. The relaxed buffer portion of the SiGe layer14 is formed utilizing a deposition process such as, for example anUHVCVD or a RTCVD, that is well known to those skilled in the art. Theupper relaxed SiGe buffer portion of the SiGe layer 14 has a thicknessthat is typically from about 500 to about 3000 nm.

The SiGe layer 14 formed as described above is a highly relaxed (greaterthan 90%) layer that has a high content (greater than 25 atomic %) ofGe. The thickness of the SiGe layer 14 is the total thickness of thegraded portion and the upper relaxed SiGe buffer portion. Typically, andusing the numbers provided above, the SiGe layer 14 has a thickness fromabout 1 to about 5 μm. It is noted that in FIG. 1A the differentportions (i.e., lower graded portion and upper relaxed SiGe bufferportion) are not specifically shown. If shown, the lower portion of theSiGe layer 14 that is in close proximity to the surface of sacrificialsubstrate 12 would be comprised of a low Ge content region, the areaabove the low Ge content area would comprise a region in which the Gecontent was increased in a stepwise fashion and the upper portion atopthe increased Ge content step portion would comprise the relaxed SiGebuffer region.

Since the surface of the SiGe layer 14 is typically rough, having apeak-to-peak roughness R_(max) in the range from 40 to 100 nm, aplanarization process such as chemical mechanical polishing (CMP) istypically required to smooth and thin the SiGe layer 14 at this point ofthe present invention. Thus, the structure shown in FIG. 1A can now besubjected to a CMP process that is capable of smoothing and thinning theSiGe layer 14. In order to alleviate or eliminate the embedded particleproblem during the planarization process, which results in difficulty inremoving particles generated by a conventional CMP process in thesubsequent cleaning steps, a non-conventional low-down force of about 1to 2 psi is employed in the present invention. As stated above, theplanarization step, provides a structure in which the SiGe layer 14 issmooth, i.e., having a R_(max) of less than 10 nm, and thin, having athickness from about 500 to about 1000 nm.

After the planarization process, the structure 10, particularly layer14, is typically subjected to a cleaning step that is capable ofremoving unwanted particles from the surface of the SiGe layer 14. Theunwanted particles are generated from the CMP process mentioned above.Although any cleaning process can be employed, a modified RCA wetcleaning process is employed in the present invention. The modified RCAprocess employs a combination of NH₄OH (ammonia hydroxide), H₂O₂(hydrogen peroxide) and deionized water (DI) in which the ratio of thecomponents is 1:1:5. This combination, which is heated at 50° C.-60° C.,can thoroughly clean residue (including particles) left from CMP on SiGebuffer wafers. The modified RCA clean process is preferred in thepresent invention since it is capable of forming a hydrophilic surfacethat is terminated with hydroxyl groups. The cleaning process mayinclude a single cleaning step, but multiple cleaning steps arepreferred.

After providing a sufficiently clean and particle free SiGe surface, alow temperature oxide 16 is formed atop the SiGe layer 14. In thepresent invention, the low temperature oxide 16 can be formed directlyon the SiGe layer 14 or a high temperature oxide can first be formed andthen the low temperature oxide 16 is formed on the high temperatureoxide. The low temperature oxide 16 is formed by either a plasmaenhanced chemical vapor deposition (PECVD) process or a low pressurechemical vapor deposition (LPCVD) process, both of which are well knownto those skilled in the art. The low temperature oxide is formed at adeposition temperature of about 450° C. or less. The low temperatureoxide (hereinafter LTO) 16 formed at this point of the present inventionhas a thickness that typically ranges from about 200 to about 400 nm.More preferably, the LTO 16 has a thickness from about 280 to about 320nm. As known to those skilled in the art, a LTO is characterized ashaving an amorphous crystal structure.

When a high temperature oxide (HTO) is employed, the HTO is firstformed, followed by the formation of the LTO 16. In those embodiments inwhich a HTO is formed, the HTO is formed by a rapid thermal chemicalvapor deposition process or any other deposition process in which thedeposition temperature is greater than 500° C. If present, the HTO has athickness that typically ranges from about 5 to about 30 nm. Morepreferably, the HTO would have a thickness from about 10 to about 20 nm.As known to those skilled in the art, HTO may be a crystalline oxide atcertain critical thickness, of less than about 5 nm.

It should be noted that in the drawings of the present invention, theHTO layer is not shown. In the embodiments in which it is present, theHTO would be located between the SiGe layer 14 and the overlying LTO 16.The presence of the HTO helps in tailoring the thickness of theinterfacial bonding layer that will be subsequently formed.

Next, a first annealing step is performed at a first annealingtemperature T1 which is capable of forming an interfacial bonding layer18 between the LTO 16 and the SiGe layer 14. In embodiments in which theHTO is present, the interfacial bonding layer 18 is located between theHTO/LTO stack and the SiGe layer 14. In accordance with the presentinvention, the interfacial bonding layer 18 comprises elements of Si, Geand O. Thus, the interfacial layer 18 is referred to herein as a SiGeOlayer.

The first annealing step employed in the present invention is performedat a temperature T1 from about 600° to about 700° C. for a period oftime from about 300 to about 1000 minutes. More preferably, the firstannealing step which forms the interfacial layer 18 is performed at atemperature T1 from about 620° to about 630° C. for a period of timefrom about 450 to about 800 minutes. The first annealing step istypically performed in an inert ambient including He, N₂, Ar, Kr, Ne, Xeor mixtures thereof. Alternatively, a forming gas which includes amixture of N₂ and H₂ can be employed. The first anneal may be performedat a single targeted temperature utilizing a single ramp up rate, orvarious ramp and soak cycles using various ramp rates and soak times canbe employed.

The first anneal also serves the purpose of driving out excessivehydroxyl groups and volatile gases inside the LTO 16, as well asdensifying the LTO 16. Adjusting the annealing temperature and/orvarying the annealing time can tailor the thickness of the interfaciallayer 18. Typically, the interfacial layer 18 formed by the firstannealing step of the present invention has a thickness from about 10 toabout 50 nm, with a thickness from about 25 to about 35 nm being moretypical.

In addition to the above features, the interfacial layer 18 is crucialfor stopping any outgas from the SiGe layer 14 during the post-bondingand the splitting anneals mentioned herein below so as to guarantee theformation of a substantially low defect and high quality SGOI substrate.In essence, the interfacial layer 18 helps to “getter” structuraldefects and any residual gases promoting a stronger and more stableinterface between the SiGe layer 14 and the LTO 16. The resultantstructure that is formed after the first annealing step has beenperformed is shown, for example, in FIG. 1B.

Next, an implant region 20 is formed within a region of the SiGe layer14 as is shown in FIG. 1C. The implant region 20 is formed by implantingH ions 22 such as H₂ ⁺ through the LTO 16, the interfacial layer 18 intothe SiGe layer 14. The implant region 20 has a peak ion concentrationthat is located at a depth from about 200 to about 500 nm below theupper surface of the SiGe layer 14. The implant region 20 is formed byimplantation of hydrogen ions. The implant conditions can vary dependingupon the thickness of the SiGe layer 14. Typical implantation conditionsused in forming the implant region 20 are as follows: ion energy fromabout 60 to about 150 KeV and a hydrogen ion dose from about 3E16 toabout 5E16 atoms/cm². More typically, the implant region 20 is formedutilizing an ion implantation process that is performed at an energyfrom about 120 to about 125 KeV and a hydrogen ion dose from about3.5E16 to about 4.5E16 atoms/cm².

Next, another CMP step can be used to trim down the LTO 16 thickness toa desired thickness value that can be chosen by those skilled in theart. Typically, and for most types of devices, the LTO 16 is trimmeddown at this point of the present invention to a thickness from about100 to about 200 nm, with a thickness from about 140 to about 160 nmbeing more typical. The CMP process performed at this point of theinventive method also serves to achieve a smooth LTO surface which meetsthe typical surface requirements for wafer bonding, i.e., the root meansquare roughness R_(rms) is less than 0.5 nm. After the CMP process,further cleaning steps as described above can be employed to clean thesmoothed surface of the LTO 16.

Next, the structure shown in FIG. 1C is bonded to a semiconductorsubstrate 24 providing the structure shown for example, in FIG. 1D. Asshown, the exposed LTO surface shown in FIG. 1C is bonded to a surfaceof the semiconductor substrate 24. The semiconductor substrate 24includes one of the above mentioned semiconducting materials that can beused as the sacrificial substrate 12. The bonding step includes firstbringing a surface of semiconductor substrate 24 into intimate contactwith a surface of the LTO 16. An external pressure can be applied duringand/or after the contact step.

The contact and thus the initial bonding are performed at nominal roomtemperature. By “nominal room temperature” it is meant a temperaturefrom about 18° to about 40° C. Prior to the bonding process, the surfaceof semiconductor substrate 24 which will be bonded to the LTO 16 issubjected to a cleaning process such as the modified RCA cleaningprocess mentioned above. It is noted that the modified RCA clean processforms a hydrophilic surface that is terminated by hydroxyl groups. Theformation of hydrophilic surfaces on both the LTO 16 and thesemiconductor substrate 24 helps to facilitate the bonding between thetwo layers. Specifically, the bonding is facilitated at these matingsurfaces (the LTO 16 and the semiconductor substrate 24) by theformation of hydrogen bonds and the subsequent formation of strongersiloxane bonds.

After the initial bonding process which includes contact bonding, thebonding process further includes a post-bonding anneal (i.e., a secondannealing step) that is performed at a temperature T2 that is relativelylow so as to prevent hydrogen induced crack propagation in the implantregion 20 from occurring prior to bond strengthen which is achievedduring this post-annealing (i.e., second annealing). Typically, thesecond annealing step is performed at a temperature T2 from about 225°to about 350° C. for a time period from about 5 to about 30 hours. Moretypically, the second annealing step is performed at a temperature T2from about 250° to about 300° C. for a time period from about 16 toabout 24 hours. This annealing step is performed in one of the abovementioned ambients and various heating regimes including different rampup rates, soak cycles and cool down rates can be employed

Following the bonding strengthen second annealing step, a thirdannealing step is performed at a temperature T3 which is greater than T2so as to allow a hydrogen induced Oswald ripen effect to occur, i.e., toform a crack in the SiGe layer 14 at the plane of the implant region 20.That is, T3 is performed at a temperature that forms a crack at theimplant region 20 which is capable of separating, i.e., splitting, aportion of the SiGe layer 14 and the underlying sacrificial substrate 12from the structure. A razor blade or other like means can be used to aidin the separation process. The resultant structure is shown, for examplein FIG. 1E.

Typically, the third annealing step (which can be referred to as asplitting anneal) is performed at a temperature T3 from about 485° toabout 550° C. for a time period from about 4 to about 6 hours. Moretypically, the third annealing step is performed at a temperature T3from about 495° to about 505° C. for a time period from about 4.5 toabout 5.5 hours. This third annealing step is performed in one of theabove mentioned ambients and various heating regimes including differentramp up rates, soak cycles and cool down rates can be employed.

In accordance with the present invention, the temperature T2 tostrengthen the bonded pair is less than the temperature T3 to causesplitting of the structure. Moreover, temperature T1 used in forming thebonding interface between the LTO 16 and the filly relaxed SiGe layer 14is greater than the splitting temperature T3. Also, temperature T1 isequal to or greater than T4 (which is discussed below).

A re-annealing step that is performed at a fourth temperature T4 that isgreater than T3 can be performed to further strengthen the bondingbetween the layers. The re-annealing step, i.e., fourth anneal performedin the present invention, is typically conducted at a temperature T4from about 600° to about 700° C. for a time period from about 1 to about10 hours. More typically, the fourth annealing step is performed at atemperature T4 from about 620° to about 630° C. for a time period fromabout 7.5 to about 8.5 hours. This fourth annealing step is performed inone of the above mentioned ambients and various heating regimesincluding different ramp up rates, soak cycles and cool down rates canbe employed.

Note that the second and third annealing steps may be performed withoutbreaking vacuum within a same annealing chamber. Alternatively, thesecond, third and fourth anneals may be performed in different annealingchambers, if desired.

At this point of the present invention, the SiGe layer 14 that remainsafter the splitting process can be subjected to a thinning step whereinCMP, ion beam etching, or a high pressure oxidation and a wet etchprocess can be used to thin the SiGe layer 14 to a desired finalthickness. Typically, the desired final thickness for the SiGe layer 14is from about 5 to about 50 nm, with a final desired thickness of fromabout 10 to about 25 nm being more typical. Prior to thinning, thesurface of the remaining SiGe layer 14 can be smoothed utilizing the lowdown force CMP process mentioned above.

The above processing steps provide a SiGe-on-insulator (SGOI) substrate26 (see FIG. 1E) that comprises the semiconductor substrate 24, the lowtemperature oxide 16 located on the semiconductor substrate 24, and thefully relaxed SiGe layer 14 having a defect density of about 10⁴ toabout 10⁵ defects/cm² or less and a Ge content that is greater than 25atomic % located atop the low temperature oxide 16. As shown, theinterfacial layer 18 is still present between the low temperature oxide16 and the fully relaxed SiGe layer 14. Note that the LTO 16 is theburied insulating layer of the final SGOI substrate 26.

FIG. 2 shows a further processing step in which a strained semiconductorlayer 28 is formed on the SiGe layer 14 or alternatively a thin (on theorder of about 10 nm or less) regrown SiGe layer with either the same Gecontent as layer 14 or a different Ge content can be formed prior toformation of the strained semiconductor layer 28. The regrown SiGe layeris formed as described above. The strained semiconductor layer 28 whichcan be comprised of Si, SiGe, SiC, SiGeC and the like is formed by anepitaxial growth process. The strained semiconductor layer 28 formed atthis point of the present invention typically has a thickness from about2 to about 20 nm, with a thickness from about 3 to about 10 nm beingmore typical.

It should be noted that the surface crystal orientation of layer 14 andthus the strained semiconductor layer 28 can be (100), (110), (111) orany other like crystallographic orientation.

Conventional CMOS processing steps can then be performed to provide atleast one CMOS device such as a FET on the surface of the structureshown in FIG. 2.

The following example is provided to illustrate the method of thepresent invention which is used in forming a substantially defect free,high Ge content SGOI substrate.

EXAMPLE

In this example, a substantially low defect, high Ge content SGOI waferis provided utilizing the processing steps of the present invention.This process follows the general procedure outlined above, but providesa more detailed and specific implementation of the inventive method. Theprocess starts with a bulk Si wafer that was cleaned by conventional RCAwet cleans. A graded SiGe buffer layer was then deposited on the bulk Siwafer utilizing a conventional deposition process. The total thicknessof the SiGe buffer layer was about 1.5 to 2.0 μm. Chemical mechanicalpolishing (CMP) with a down-force of about 1.5 psi was used to smooththe surface of the SiGe buffer layer and simultaneously to reduce thethickness of the buffer layer by an amount of a couple of hundrednanometers. Afterwards, and in addition to the conventional brushcleaning normally associated with CMP, the wafer was further cleaned ina modified RCA solution several times to remove the particles and theresiduals left from CMP. Once the wafer reached the required cleanness(usually several tens to a couple of hundreds particles counted by aparticle scanner on the wafer are acceptable), a LTO having a thicknessof about 300 nm was deposited on top of the smoothed and cleaned SiGelayer. In order to eliminate defects induced by outgassing in thesubsequent process steps involving elevated temperatures, the wafer withthe deposited LTO was annealed at 625° C. for about 800 minutes. The TEMcross section shown in FIG. 3 indicated that a SiGeO interfacial oxidelayer with a thickness of about 30 to 40 nm was formed between the LTOand the SiGe buffer layer. In FIG. 3, the Si handlesubstrate=semiconductor substrate 24, BOX=LTO 16, SiGeO=interface 18,t-SiGe=SiGe layer 14, and the remaining layers are additional layers, asindicated, that can be formed atop the SGOI substrate of the presentapplication.

The wafer was then ion-implanted with ionized hydrogen H₂ ⁺ at an energyof about 130 KeV and up to a dose of about 3.6E16 atoms/cm². Based onSIMS data (not shown), the ionized hydrogen at this energy penetratedthrough the LTO layer and peaked at a depth of about 300 nm into theSiGe buffer layer. After hydrogen implantation, the LTO layer waspolished to reduce its thickness down to a designated thickness of about150 nm as well as to smooth its surface so as to satisfy the surfacecondition for bonding, i.e., R_(rms)<0.5 nm. The particulate residuefrom the CMP was cleaned using a brush clean followed by a modified RCAclean. The surface of the LTO and a Si handle substrate were recleanedin a fresh modified RCA clean solution, and the two bonding surfaceswere dried in a N₂ ambient prior to bringing the two surfaces incontact.

Post-bonding annealing was conducted at 300° C. for 20 hours tostrengthen the bond between the LTO and Si wafer by converting thehydrogen bond to a covalent one. The bonded pair was split by annealingat 500° C. for 5 hours. The handle wafer with the transferred layer wasthen annealed at 625° C. for 8 hours to further strengthen the integrityof SGOI structure. CMP touch polish was used to smooth the transferredSGOI layer. The final SGOI thickness can be further reduced by eitherCMP or ion-beam etching.

Mobility experiments were performed for strained Si-MODFET devices thatwere built on such a SGOI substrate and the results of these experimentsare summarized as follows:

at 295 K: 1741 cm²/Vs, 1.46×10¹² cm⁻²;

at 25 K: 16,062 cm²/Vs, 1.26×10¹² cm⁻².

The results from the mobility experiments demonstrated that highmobility can be achieved using the inventive SGOI substrate material.Similarly, for current strained Si MOSFET devices electron mobility inthe range of 500-1000 cm²/Vs has been demonstrated on SGOI substrateswith Ge contents from 20-50 atomic %.

For n-MOSFET or n-MODFET device applications, epitaxial silicon can bedeposited on top of the final SGOI structure to form a tensilelystrained silicon layer for enhanced electron mobility. Similarly, a highGe content SiGe layer (i.e., greater than 50%) may be deposited on topof the final SGOI structure to form a compressively strained SiGechannel for enhanced hole mobility suitable for p-MOSFET or p-MODFETdevice applications.

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof it will be understood byone skilled in the art that the foregoing and other changes in forms anddetails may be made without departing from the spirit and scope of theinvention. It is therefore intended that the present invention is notlimited to the exact forms and details described and illustrated, butfalls within the spirit and scope of the appended claims.

1. A method of forming a SiGe-on-insulator substrate comprising: forminga low temperature oxide at a deposition temperature of about 450° C. orless atop a structure that comprises a fully relaxed SiGe layer having apeak-to-peak roughness of less than 10 nm and a thickness from about 500to about 100 nm located on a sacrificial substrate, said fully relaxedSiGe layer further has a measured relaxation value of greater than 90%and comprises a lowered graded portion and an upper relaxed portion,wherein said lower graded portion comprises a low Ge content regionhaving 10 atomic % or less Ge in proximity to a surface of saidsacrificial substrate, a middle region in which the Ge content increasesin a stepwise manner, and an upper region having a Ge content that isgreater than 25 atomic %; first annealing said structure including saidlow temperature oxide at a first temperature of from about 600° to about700° C. for a time period from about 300 to about 1000 minutes to forman interfacial layer comprising elements of Si, Ge and O between the lowtemperature oxide and said SiGe layer; providing an implant region byion implanting hydrogen within said fully relaxed SiGe layer; cleaningsaid low temperature oxide and a semiconductor substrate within an agentthat forms a hydrophilic surface on each material; bonding saidhydrophilic surface of said low temperature oxide to said hydropholicsurface of said semiconductor substrate, wherein said bonding comprisescontact bonding at nominal room temperature to form a bond between saidhydrophilic surface of said low temperature oxide and said hydrophilicsurface of said semiconductor substrate, a second anneal at a secondtemperature of from about 225° to about 350° C. to strengthen said bond,and a third anneal performed at a third temperature that is greater thanthe second temperature and of from about 485° to about 550° C. to causeseparation at said implant region within said fully relaxed SiGe layer,whereby said sacrificial substrate and a portion of the fully relaxedSiGe layer are removed, and wherein said second and third annealing areperformed without breaking a vacuum; and re-annealing the structure at afourth temperature that is greater than the third temperature and isfrom about 600° to about 700° C. to form a SiGe-on-insulator (SGOI)substrate that comprises the semiconductor substrate, said lowtemperature oxide located on said semiconductor substrate, and saidfully relaxed SiGe layer having a defect density of about 10⁴ to about10⁵ defects/cm² or less and a Ge content that is greater than 25 atomic% located atop said low temperature oxide, wherein said low temperatureoxide and said fully relaxed SiGe layer are separated by saidinterfacial layer; thinning the fully relaxed SiGe layer, said thinningstep comprises chemical mechanical polishing, ion beam etching or acombination of high pressure oxidation and wet etching; and forming astrained semiconductor layer atop the fully relaxed SiGe layer.